Instruments used in analytical and preparative chemistry often include valves to control the flow of fluids. Check valves are one important class of flow-control valves and are typically utilized to permit fluid flow in one direction while impeding fluid flow in the reverse direction. Liquid chromatography systems, for example, typically include check valves in fluid communication with respective inlet and outlet ports of a displacement pump to control the direction of fluid flow through the pump.
For example, U.S. Pat. No. 3,810,716 to Abrahams et al. describes inlet and outlet check valves applied to a high-pressure chromatography reciprocating two-piston pump. The pump employs an inlet and an outlet check valve respectively on each of two displacement chambers or cylinders. The pump has a parallel configuration which delivers fluid to a downstream receiving system alternately from two pump cylinders; each cylinder is in direct fluid communication with the receiving system during a portion of the pump cycle. The alternating pattern of fluid delivery allows one pump cylinder to be refilled while delivery from the other pump cylinder sustains the desired pump output flow rate to the downstream system.
The inlet and outlet check valves associated with each cylinder in this parallel configuration allow each cylinder to communicate either with an inlet fluid pathway from a solvent reservoir or with an outlet fluid pathway to the downstream receiving system. The operation and location of the check valves substantially prevents the backflow of fluid from the pressurized system into a pump cylinder that is undergoing refill at substantially atmospheric pressure.
Another common configuration of a high-pressure pump for liquid chromatography is a serial configuration described, for example, in U.S. Pat. No. 4,245,963 to Hutchins et al. Liquid inspired at a pump intake is directed serially through a first pump cylinder and a second pump cylinder. Only a single inlet and a single outlet check valve are utilized. The coordinated motion of respective first and second pistons interacts cooperatively with the actions of the inlet and the outlet check valves to achieve a substantially constant output flow rate of liquid to a downstream receiving system.
As with the parallel pump configuration, a single drive motor may be used to affect the coordinated motion of the two pistons using appropriate gearing or equivalent drive elements. Alternatively, a separate motor may be allocated for the drive of each piston, which can allow greater operational flexibility in motion coordination.
A ball-and-seat type of chromatography pump check valve is particularly common. A typical configuration employs a stationary seat and a ball that is capable of being displaced toward or away from the seat. In a passive ball-and-seat check valve, fluid flow in one direction urges the ball against the seat, blocking the flow of fluid. Fluid flow in the opposite direction urges the ball away from the seat, opening a pathway through the valve. Some passive check valves include a spring that holds the ball against the seat until a differential fluid pressure across the valve exceeds a threshold pressure determined by the force applied by the spring. This differential pressure is often referred to as the “cracking pressure.” A low cracking pressure allows for improved pump performance because any pressure differential results in a flow disturbance when the check valve is open. Generally, the flow disturbance is more pronounced with greater cracking pressure.
The ball for a chromatography pump check valve is commonly fabricated from ruby while the seat is commonly fabricated from sapphire (both ruby and sapphire are forms of crystalline aluminum oxide.) Alternatively, check valve balls and seats are fabricated from, for example, aluminum-oxide based ceramics. These materials are chosen for desired properties, for example, for their chemical inertness, resistance to wear, machinability, and/or stiffness.
Chromatography pumps are generally high-precision devices, designed to produce substantially stable and reproducible solvent flows at delivery pressures of as much as thousands of pounds per square inch (psi) or greater (i.e., tens of megaPascals (MPa) or greater). Back-leakage of a check valve can degrade or destroy the desired relationship between a control input (such as a pump step-motor step rate or step count) and a volume delivery output of chromatography solvent. Therefore, considerable effort has been expended by ball-and-seat check valve manufacturers to produce balls that are highly spherical and that have an excellent surface finish. A corresponding effort has been expended by manufacturers to produce seats having a spherical or conical sealing surface with a close tolerance as well as an excellent surface finish.
Though providing a good seal when new, many conventional valves become fouled in service by foreign matter that lodges on either the ball or the seat sealing surface. The fouling at times is transient, where the fouling substance is swept downstream on a subsequent valve actuation or more permanent, where restoration of proper valve function may require valve disassembly and aggressive cleaning or component replacement.
While fouling matter resides on the ball or seat, the check valve may be rendered partially or wholly inoperable due to a failure to properly seal against reverse flow. In another failure mode, the ruby and sapphire materials may become stuck together in the presence of particular solvents, rendering the valve inoperable because it cannot enable fluid flow in the forward direction.
U.S. Pat. No. 8,671,975 to Charlton et al. describes a check valve that uses a compliant member on a moveable member in which a coil is used to apply a force to urge the moveable member toward a valve seat. Pressure may further assist in maintaining a seal when the valve is in a closed state. The cracking pressure that is achievable in the disclosed valve may have a cracking pressure high enough to introduce pressure disturbances that are unacceptable in some instance.
Typical check valves require axial flow of the fluid through the device and therefore cannot accommodate configurations in which the direction of flow changes, for example, when inlet and outlet flow paths are not coaxial. Some ball and seat check valves are required to be oriented vertically such that gravity locates the ball in the seat under conditions without fluid flow. Moreover, such valves are susceptible to debris which may be disposed between the ball and the seat and which prevents proper seating necessary for sealing.